This invention relates to a method for preparing a sintered body containing cubic boron nitride and a method for preparing a cubic boron nitride, more particularly to a method for preparing a cubic boron nitride-containing sintered body which contains at least cubic boron nitride as one of its constitutional components and a method for preparing cubic boron nitride under high-pressure and high-temperature conditions.
With regard to methods for preparing the cubic boron nitride sintered body, there have been made many suggestions, and these methods can roughly be classified into two manners. One of them comprises first synthesizing cubic boron nitride (hereinafter referred to as c-BN) at a high pressure and temperature, mixing the synthesized c-BN with a metal and/or compound which will serve as a binder, and preparing a sintered body from the resulting mixture at a high pressure and high-temperature again; and another of the above two manners comprises mixing boron nitride (hereinafter referred generically to as BN) such as amorphous boron nitride (hereinafter referred to as a-BN) or hexagonal boron nirtride (hereinafter referred to as h-BN), which is a starting material, with a metal and/or compound which will serve as a binder, and converting said BN into c-BN at a high pressure and high temperature and simultaneously forming a desired sintered body.
When both the manners mentioned above are compared, economically, the latter seems more preferable because of employing the only one high pressure and temperature process, though the former requires this high-cost process twice. In fact, however, the former has been utilized more prevalently in industrial fields. This reason is that in the case of the latter, the conversion of BN into c-BN will scarcely be carried out at 100% and the remaining BN will noticeably impair properties and performances of the prepared sintered body.
For the purpose of improving the conversion of BN into c-BN, methods in which catalysts are employed have been suggested. For example, there are known a method comprising first adding Co as a catalyst to h-BN as a starting material, and converting this h-BN into c-BN through a high-pressure and high-temperature reaction and simultaneously sintering it, as disclosed in Japanese Patent Publication No. 16199/1976, and a method which comprises using AlN and one or more, as a catalyst, selected from the group consisting of elements in groups Ib, IIb, IVa, Va, VIa, VIIa and VIII of the periodic table and silicon, and sintering it under a condition of a high-pressure and high-temperature, as disclosed in Japanese Patent Publication No. 17838/1977.
However, the boron nitride sintered bodies containing c-BN as a main constitutional phase prepared according to these methods are both poor in hardness, because the used metallic element is present as a binder component; therefore these sintered bodies are characteristically unsatisfactory when used as tool materials. Further, also in other applications, they are disadvantageously poor in thermal conductivity, chemical stability and the like.
The inventors of this invention have previously suggested a method for preparing the c-BN sintered body in which h-BN containing no metallic catalyst is used as a starting material, with the intention of overcoming the above-mentioned drawbacks [see Japanese Provisional Patent Publication No. 32771/1980 (Japanese Patent Publication No. 34429/1983)]. This method comprises mixing AlN with h-BN which is a starting material, which AlN works as a catalyst in a conversion reaction of h-BN into c-BN; filling a suitable container with the mixture; pouring an organic solvent such as xylene, toluene or ethyl alcohol into the container in order to adjust a content of oxygen in the container to 2% by volume or less; subjecting the material to the atmosphere of an ultra-high pressure and high-temperature, thereby converting the material h-BN into c-BN and simultaneously obtaining a sintered body. According to this method, the used organic solvent will release bonded hydrogen in the ultra-high pressure and high-temperature treatment process. This nascent hydrogen will remove harmful oxygen which is physically and chemically present in the starting material h-BN, in order to permit completely converting h-BN into c-BN with the aid of the catalytic action of AlN and simultaneously forming the desired sintered body in which the c-BN components are combined with each other.
However, the inventors of this invention have found from additional deep researches that the sintered body prepared according to the above-mentioned method has the following drawbacks. That is, an organic solvent such as xylene or toluene will behave as described above to eliminate deleterious oxygen present in the starting material BN and will contribute to the conversion of the BN into c-BN and the formation of the sintered body, but the solvent will leave free carbon as a residue which will prevent the c-BN components from combining with each other, with the result that the sintered body will be poor in strength.